Coatings using filled hydrogen silsequioxane

Abstract: The present invention relates to a method of forming coatings on substrates and the substrates coated thereby. The method comprises applying a coating comprising hydrogen silsesquioxane resin and a filler on a substrate and heating the coated substrate at a temperature sufficient to convert the hydrogen silsesquioxane resin to a ceramic coating. This method is especially valuable for forming coatings on electronic devices.

Description: The present invention relates to a method of forming coatings using compositions comprising hydrogen silsesquioxane resin and fillers. These coatings are useful on substrates for electronic devices. The use of hydrogen silsesquioxane resins (H-resins) to derive ceramic coatings on substrates such as electronic devices is known in the art. For instance, U.S. Patent No. 4,756,977 discloses a process for forming a silica coating on an electronic substrate wherein a solution of H-resin is applied to a substrate followed by heating the coated substrate in air at a temperature in the range of 200-1000°C. This reference, however, does not describe the use of fillers within the coating. However, the use of fillers within ceramic coatings is also known in the art. For instance, U.S. Patent No. 3,986,997 describes a composition comprising an acidic dispersion of colloidal silica and hydroxylated silsesquioxane in an alcohol-water medium which can be used to apply transparent abrasion resistant coatings on a variety of substrates. The reference, however, does not describe the use of H-resins nor the application of a ceramic coating on an electronic substrate. We have now found that useful protective coatings of ceramics can be formed from compositions comprising H-resins and fillers. The present invention relates to a method of forming a coating on an electronic substrate and the substrates coated thereby. The method comprises first applying a composition comprising H-resin and a filler on the electronic substrate. The coated substrate is then heated at a temperature of 50 to 1000°C. sufficient to convert the composition to a ceramic coating. The present invention also relates to the coating composition comprising H-resin and the filler diluted in a solvent that is used to form the coatings. The present invention is based on our finding that desirable coatings can be formed from a composition comprising H-resin and a filler. Coatings derived therefrom can be thicker (e.g., greater than 40 micrometers) compared to those formerly derived from unfilled hydrogen silsesquioxane resins (e.g., less than 2 micrometers). They can also have a variety of electrical properties depending on the coating (e.g. from coatings with a dielectric constant (DK) of less than 4 up to completely conductive coatings) and they can be crack and pore-free. In addition, high temperature firing of the coatings to burn-out carbon is not necessary since H-resin is nearly carbon-free. Because of the above advantages, these coatings are particularly valuable on electronic substrates. Such coatings could serve as protective or dielectric coatings, conductive coatings, interlevel dielectric layers and doped dielectric layers to produce transistor like devices, pigment loaded binder systems containing silicon to produce capacitor and capacitor like devices, multilayer devices, 3-D devices, silicon on insulator devices, coatings for superconductors and super lattice devices. The choice of substrates to be coated by the instant invention, however, is limited only by the need for thermal and chemical stability of the substrate under the conditions used. As used herein, the expression "ceramic coating" is used to describe the hard coating obtained after heating the hydrogen silsesquioxane resin - filler composition. This coating contains both amorphous silica (SiO 2 ) materials as well as amorphous silica-like materials that are not fully free of residual carbon, silanol (Si-OH) and/or hydrogen (which are obtained upon heating the H-resin) and filler materials. The expression "filler" is used to describe a finely divided solid phase which is distributed within the H-resin and the final ceramic coating. The expression "electronic substrate" includes electronic devices or electronic circuits such as silicon based devices, gallium arsenide based devices, focal plane arrays, opto-electronic devices, photovoltaic cells and optical devices. In the process of the present invention a ceramic coating is formed on an electronic substrate by a process which comprises applying a coating composition comprising H-resin and a filler onto the substrate and then heating the coated substrate at a temperature sufficient to convert the composition to a ceramic. The H-resins which may be used in this invention include hydridosiloxane resins of the formula HSi(OH) x (OR) y O z/2 , in which each R is independently an organic group or a substituted organic group which, when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent, x = 0-2, y = 0-2, z = 1-3, x + y + z = 3. Examples of R include alkyls such as methyl, ethyl, propyl and butyl; aryls such as phenyl; and alkenyls such as allyl or vinyl. As such, these resins may be fully condensed (HSi0 3/2 ) n or they may be only partially hydrolyzed (i.e., containing some Si-OR) and/or partially condensed (i.e., containing some Si-OH). Although not represented by this structure, these resins may contain a small number (eg., less than 10%) of silicon atoms which have either 0 or 2 hydrogen atoms attached thereto due to various factors involved in their formation or handling. The above H-resins and methods for their production are known in the art. For example, U.S. Patent No. 3,615,272 teaches the production of a nearly fully condensed H-resin (which may contain up to 100-300 ppm silanol) by a process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium and then washing the resultant resin with water or aqueous sulfuric acid. Similarly, U.S. Patent No. 5,010,159 provides an alternative method comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin which is then contacted with a neutralizing agent. Other hydridosiloxane resins, such as U.S. Patent No. 4,999,397 are produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium. Other equivalent hydridosiloxanes will also function herein such as JP-A 59-178749, 60-86017 and 63-107122. In a preferred embodiment, specific molecular weight fractions of the above H-resins may also be used in our process. Such fraction and methods for their preparation are conveniently taught in U.S. Patent No. 5,063,267. A preferred fraction comprises material wherein at least 75% of the polymeric species have a weight average molecular weight above 1200 and a more preferred fraction comprises material wherein at least 75% of the polymeric species have a weight average molecular weight between 1200 and 100,000. The coating composition may also contain ceramic oxide precursors. Examples of such ceramic oxide precursors include compounds of various metals such as aluminum, titanium, zirconium, tantalum, niobium and/or vanadium as well as various non-metallic compounds such as those of boron or phosphorous which may be dissolved in solution, hydrolyzed and subsequently pyrolyzed, at relatively low temperatures and relatively rapid reaction rates to form ceramic oxide coatings. The above ceramic oxide precursor compounds generally have one or more hydrolyzable groups bonded to the above metal or non-metal, depending on the valence of the metal. The number of hydrolyzable groups to be included in these compounds is not critical as long as the compound is soluble in the solvent. Likewise, selection of the exact hydrolyzable substituent is not critical since the substituents are either hydrolyzed or pyrolyzed out of the system. Typical hydrolyzable groups include alkoxy such as methoxy, propoxy, butoxy and hexoxy; acyloxy such as acetoxy; or other organic groups bonded to said metal or non-metal through an oxygen such as acetylacetonate. Specific compounds, therefore, include zirconium tetracetylacetonate, titanium dibutoxy diacetylacetonate, aluminum triacetylacetonate and tetraisobutoxy titanium. When hydrogen silsesquioxane resin is to be combined with one of the above ceramic oxide precursors, generally it is used in an amount such that the final ceramic coating contains 0.1 to 30 percent by weight of modifying ceramic oxide. The coating composition may also contain a platinum, rhodium or copper catalyst to increase the rate and extent of conversion to silica. Generally, any platinum, rhodium or copper compound or complex which can be solubilized will be functional. For instance, a composition such as platinum acetylacetonate, rhodium catalyst RhCl 3 [S(CH 2 CH 2 CH 2 CH 3 ) 2 ] 3 , obtained from Dow Corning Corporation, Midland, Michigan, or cupric naphthenate are of this invention. These catalysts are generally added in an amount of between about 5 to 1000 ppm platinum, rhodium or copper based on the weight of H-resin. The fillers used herein are known in the art for use in coatings with other polymers. These include various inorganic and organic fillers, especially inorganic fillers, in a variety of morphologies including powders, particles, flakes and microballoons. Examples of inorganic fillers include synthetic and natural materials such as the oxides, nitrides, borides and carbides of various metals and non-metals such as glass, alumina, silica, titanium dioxide, zinc oxide, tungsten oxide and ruthenium oxide; titanates such as potassium titanate and barium titanate; niobates such as lithium niobate and lead niobate; barium sulfate; calcium carbonate; precipitated diatomite; aluminum silicate or other silicates; pigments; phosphors; wollastonite; mica; kaolin; clay and talc. Also, some organic materials such as cellulose, polyamides and phenol resins may also be used. The preferred fillers to be used herein depend on the intended use for the coating. For instance, if the coating is to be used as an interlevel dielectric, a material such as silica or alumina is desirable so that the coating has a low dielectric constant (DK). Similarly, if a coating having a high DK is desired, a material such as barium titantate or lead niobate may be desirable. The particle size and shape of the above fillers can vary over a wide range depending on factors such as the type of filler and the desired coating thickness. Since the coatings are generally less than 500 micrometers thick, particle sizes less than this are generally used. Preferred size particles are generally in the range of less than 100 micrometers, more preferred particle sizes are in th range of less than 50 micrometers and the most preferred particles are in the range of submicrometer to 10 micrometers. The amount of filler used in the present invention can also be varied over a wide range depending, for example, on the quality and electrical characteristics desired in the final coating. Generally, however, the fillers are used in an amount less than 90 weight percent based on the weight of the H-resin to insure that enough resin is present to bind the filler. Obviously, smaller amounts of fillers (eg., 1-5 wt%) can also be used. If desired, other materials may also be present in the coating composition. For instance, it is within the scope of the present invention to use a material which modifies the surface of the filler for better adhesion. Such materials can include silanes such as glycidoxypropyltrimethoxysilane, mercaptopropyltrimethoxysilane and vinyltriacetoxysilane. Similarly, it is within the scope of the invention to include suspending agents in the coating composition. These and other optional components are known to those skilled in the art. According to the invention, the H-resin, filler and any optional components are applied to the surface of a substrate. This can be accomplished in any manner, but a preferred method involves dissolving the H-resin in a solvent and dispersing the filler and any optional components therein. This dispersion is then applied to the surface of the substrate. Various facilitating measures such as stirring and/or heating may be used to dissolve or disperse the H-resin and filler and create a more uniform application of the materials. Solvents which may be used include any agent or mixture of agents which will dissolve or disperse the H-resin and filler to form a homogenous liquid mixture without affecting the resultant coating. These solvents include alcohols such as ethyl or isopropyl; aromatic hydrocarbons such as benzene or toluene; alkanes such as n-heptane or dodecane; ketones; esters; glycol ethers or cyclic dimethylpolysiloxanes, in an amount sufficient to dissolve/disperse the above materials to the concentration desired for application. Generally, enough of the above solvent is used to form a 0.1-80 weight percent mixture, preferably 1-50 weight percent. If a liquid method is used, the liquid mixture comprising the H-resin, filler, solvent and any optional components is then coated onto the substrate. The method of coating can be spin coating, dip coating, spray coating or flow coating. Other equivalent means, however, are also deemed to be within the scope of this invention. The solvent is then allowed to evaporate from the coated substrate resulting in the deposition of the H-resin and filler coating. Any suitable means of evaporation may be used such as simple air drying by exposure to an ambient environment, by the application of a vacuum or mild heat (e.g., less than 50°C.) or during the early stages of the heat treatment. When spin coating is used, the additional drying period is minimized as the spinning drives off the solvent. Although the described methods focus on a liquid approach, one skilled in the art would recognize that other equivalent means would also function herein and are contemplated to be of this invention. The H-resin and filler coating is then typically converted to the ceramic by heating it to a sufficient temperature. Generally, the temperature is in the range of 50 to 1000°C. depending on the pyrolysis atmosphere. Preferred temperatures are in the range of 50 to 800°C. and, most preferably, 50-500°C. Heating is conducted for less than 3 hours to ceramify the coating. The above heating may be conducted at any effective atmospheric pressure from vacuum to superatmospheric and under any effective oxidizing or non-oxidizing gaseous environment such as those comprising air, O 2 , an inert gas (N 2 , etc.), ammonia, amines, moisture and N 2 O. Any method of heating such as the use of a convection oven, rapid thermal processing, hot plate or radiant or microwave energy is generally functional herein. The rate of heating, moreover, is also not critical, but it is most practical and preferred to heat as rapidly as possible. By the above methods a ceramic coating is produced on the substrate. The thickness of the coating can vary over a wide range (e.g., up to 500 micrometers) as described above. These coatings smooth the irregular surfaces of various substrates; they are relatively defect free; they have excellent adhesive properties and they have a variety of electrical properties (e.g., DK of less than 4 up to completely conductive coatings). As such, they are particularly useful for a variety of electronic applications such as dielectric, protective and conductive layers. Additional coatings may be applied over these coatings if desired. These include Si0 2 coatings, SiO 2 /ceramic oxide layers, silicon containing coatings, silicon carbon containing coatings, silicon nitrogen containing coatings, silicon oxygen nitrogen coatings, silicon nitrogen carbon containing coatings and/or diamond like carbon coatings. Methods for the application of such coatings are known in the art and are describe9 in U.S. Patent No. 4,756,977. An especially preferred coating is silicon carbide applied by the chemical vapor deposition of silacyclobutane. This latter process is described in U.S. Patent No. 5,011,706. The following non-limiting examples are included so that one skilled in the art may more readily understand the invention. Example 1 Eccospheres™ DCT-28-27 (silica glass microballoons with a particle size range of 10-40 micrometers), 0.2 g, were ground in a mortar and pestle for 20 minutes to decrease the particle size. A coating composition was then formed by mixing the ground glass, 0.7 g of H-resin made by the method of U.S. Patent No. 3,615,273 and 0.012 g glycidoxypropyltrimethoxysilane and 0.36 g dodecane. The coating composition was applied to the surface of two 7.6 cm x 12.7 cm (3"X5") aluminum panels; one using a 0.075 mm (3 mil) applicator to apply a 0.05 mm (2 mil) coating and the second using a 0.1 mm (4 mil) applicator to apply a 0.075 mm (3 mil) coating. The coatings were allowed to dry for 1 hour at 64°C. The coated panels were then heated at 185°C. for 1 hour and at 410°C. for 45 minutes. The dielectric constant and dissipation factor for the 0.075 mm (3 mil) coating are as follows: Frequency Dielectric Constant (DK) Dissipation Factor 100 HZ 2.43 0.106 1 KHZ 2.21 0.097 10 KHZ 2.13 0.019 100 KHZ 2.08 0.009 The spheres had a dielectric constant = 1.17 (1-8.6 GHZ) and dissipation factor = 0.001 (1-8.6 GHZ). Example 2 A coating composition was formed by mixing 0.12 g Eccospheres™ SDT-28-27 (not crushed), 0.12 g (5 micrometer) Minusil™ (silica), 0.7 g of H-resin made by the method of U.S. Patent No. 3,615,273, 0.012 g glycidoxypropyltrimethoxysilane and 0.456 g dodecane. The coating composition was applied to the surface of various substrates and processed as set out in the following table: Substrate Film Applicator Air Dry 185°C. 410°C. Thickness (micrometer) 7.6 cm x 15.2 cm Al Panel 0.075 mm 3 hr 1.75 hr 1.25 hr 20 7.6 cm x 15.2 cm Al panel 0.100 mm 3 hr 1.75 hr 1.25 hr 41 7.6 cm x 15.2 cm Al Panel 0.125 mm 3 hr 1.75 hr 1.25 hr 48 7.6 cm Si Wafer 0.075 mm 3 hr 0.75 hr 1.00 hr 24 5.1 cm sq 1000 nm gold on Nichrome™ 0.075 mm 3 hr 0.75 hr 1.00 hr 22 5.1 cm sq Si wafer - Al coated 0.15 mm 3 hr 0.75 hr 1.00 hr 42 Al interdigitated pattern on 5.1 cm diameter Si wafer 0.15 mm 3 hr 0.75 hr 1.00 hr 22 The aluminum interdigitated pattern on the silicon wafer coated above was tested for porosity, pinhole density and barrier layer properties by MIL STD 883C, Method 2021-3, Procedure B, for determining glassivation layer integrity with a phosphoric acid etch solution. The etch solution consisted of 16 parts concentrated phosphoric acid, 2 parts deionized water, 1 part concentrated nitric acid and 1 part glacial acetic acid. The sample was tested by applying 1 drop of the etch solution on the surface of the coating for a period of 50 minutes (30 minutes = normal exposure time). The sample was next rinsed with distilled water and allowed to dry. Examination of the test area on the coating showed the film was intact and there was no evidence of any corrosion. The dielectric constant and dissipation factor for the 7.6 cm x 15.2 cm (3"X6") aluminum panel coated above with the 41 micrometer thick coating are as follows: Frequency Dielectric Constant (DK) Dissipation Factor 100 HZ 2.42 0.032 1 KHZ 2.35 0.014 10 KHZ 2.32 0.006 100 KHZ 2.31 0.004 Example 3 A coating composition was formed by mixing 0.12 g Eccospheres™ SDT-28-27 (not crushed), 0.12 g plasma alumina, average particle size = 6 micrometers (Product No. 13,699, ZYP Coatings, Inc.), 0.7 g of H-resin made by the method of U.S. Patent No. 3,615,273, 0.012 g glycidoxypropyltrimethoxysilane and 0.456 g dodecane. The coating composition was applied to the surface of a 7.6 cm x 12.7 cm (3"X5") aluminum panel using a 0.075 mm (3 mil) applicator. The coatings were allowed to dry for 3 hours at 64°C. The coated panels were then heated at 185°C. for 1 hour and at 400°C. for 1 hour. A 31 micrometer thick coating was obtained. The dielectric constant and dissipation factor for the coating are as follows: Frequency Dielectric Constant (DK) Dissipation Factor 100 HZ 2.04 0.058 1 KHZ 1.94 0.030 10 KHZ 1.87 0.020 100 KHZ 1.82 0.015 Example 4 A coating composition was formed by mixing 0.15 g plasma alumina, average particle size = 6 micrometers (Product No. 13,699, ZYP Coatings, Inc.), 0.7 g of H-resin made by the method of U.S. Patent No. 3,615,273, 0.024 g glycidoxypropyltrimethoxysilane and 0.271 g dodecane. The coating composition was applied to the surface of a 7.6 cm x 12.7 cm (3"X5") aluminum panel using a 0.05mm (2 mil) applicator. The coatings were allowed to dry for 4 hours at 64°C. The coated panels were then heated at 185°C. for 1 hour and at 400°C. for 1 hour. A 36 micrometer thick coating was obtained. The dielectric constant and dissipatic factor for the coating are as follows: Frequency Dielectric Constant (DK) Dissipation Factor 100 HZ 2.23 0.038 1 KHZ 2.15 0.017 10 KHZ 2.12 0.008 100 KHZ 2.10 0.005 Example 5 Nalco™ 84SS-258 (30 % colloidal silica with a particle size of 20 nanometer diluted in a glycol propyl ether), 1.08 g, 0.6 g of H-resin made by the method of U.S. Patent No. 3,615,273 and 1.08 g 2(2-butoxyethoxy)ethyl acetate were mixed in 1.11 g methyl ethyl ketone. The coating composition was applied to the surface of 10 Motorola 14011B CMOS devices and spun at 3000 RPM for 10 seconds. A 2.5 cm (1 inch) square silicon wafer was also coated in the same manner. The coated materials were then heated at 400°C. for 2.5 hours in air. The CMOS devices were all functional after pyrolysis. Salt atmosphere tests per MIL-STD-883C, method 1009, showed that 7 out of 10 passed 2 hours and 3 of 7 passed 4 hours in the test. A similar coating of silica derived from H-resin failed after 10 minutes in the same type test. Example 6 Nalco™ 84SS-258 (30 % colloidal silica with a particle size of 20 nanometer diluted in a glycol propyl ether), 2.163 g, 2.0 g of triethoxysilane, 0.164 g water, 9.49 g isopropyl alcohol and 3.0 g n-butanol were mixed and heated at 60-75°C. for 30 minutes. The coating composition was applied to the surface of 2.5 cm x 7.6 cm (1"X3") aluminum panel. The coated panel was air dried for 10 minutes and pyrolyzed at 400°C. for 1 hour in air. A Motorola 14011B CMOS device and a 2.5 cm (1 inch) square silicon wafer were also spin coated with the above liquid mixture at 3000 RPM for 15 seconds. The coated parts were then heated at 400°C. for 4 hours in air. FTIR showed essentially complete conversion to silica. The CMOS device was functional after pyrolysis and survived 4 hrs of the salt atmosphere tests of Example 5 (failed at 24 hrs).

Claims: A method of forming a coating on an electronic substrate comprising: applying a coating composition comprising hydrogen silsesquioxane resin and a filler onto the electronic substrate; and heating the coated substrate at a temperature between 50 and 1000°C. for less than three hours to convert the coating composition into a ceramic coating. The method of claim 1 wherein the coating composition is applied to the substrate by a process which comprises coating the substrate with a liquid mixture comprising a solvent, the hydrogen silsesquioxane resin and the filler and then evaporating the solvent. The method of claim 2 wherein said liquid mixture is coated onto the substrate by spray coating, dip coating, flow coating or spin coating. The method of claim 2 wherein the coated substrate is heated in an environment selected from air, O 2 , oxygen plasma, an inert gas, ammonia, amines, moisture and N 2 O. The method of claim 1 wherein the hydrogen silsesquioxane resin is fractionated such that at least 75% of the polymeric species have a weight average molecular weight between 1200 and 100,000. The method of claim 1 wherein the coating composition also contains modifying ceramic oxide precursors comprising a compound containing an element selected from titanium, zirconium, aluminum, tantalum, vanadium, niobium, boron and phosphorous wherein the compound contains at least one hydrolyzable substituent selected from alkoxy or acyloxy and the compound is present in an amount such that the coating contains 0.1 to 30 percent by weight modifying ceramic oxide. The method of claim 1 wherein the coating composition also contains a platinum, rhodium or copper catalyst in an amount of between 5 and 1000 ppm platinum, rhodium or copper based on the weight of hydrogen silsesquioxane resin. The method of claim 1 wherein the coating composition also contains a material which modifies the surface of the filler. The method of claim 1 wherein the coating composition also contains a suspending agent. The method of claim 1 wherein the filler is in a form selected from powders, particles, flakes and microballoons. The method of claim 1 wherein the filler is selected from oxides, carbides, nitrides, borides, titanates, niobates, silicates, phosphors, wollastonite, mica, kaolin, clay, talc, cellulose, polyamides and phenol resins. The method of claim 1 wherein the filler has a particle size in the range of less than 500 micrometers. The method of claim 1 wherein the filler is present in the coating composition in an amount less than 90 weight percent. The method of claim 1 wherein the ceramic coating has a low dielectric constant. The method of claim 1 wherein the ceramic coating has a high dielectric constant. The method of claim 1 wherein the ceramic coating is conductive.

Inventors: HALUSKA LOREN ANDREW, MICHAEL KEITH WINTON, HALUSKA, LOREN ANDREW, MICHAEL, KEITH WINTON
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: C23C18
IPC codes: C03C17, C04B41, C09K11, C01B33, C23C18, C09D183, H01L23, C14C11